This invention relates generally to the protection of solid-state semiconductor devices from destructive electrostatic discharges, and more particularly to a transfer assembly interposable between a storage tube containing semiconductor devices and a processing station therefor to effect a controlled discharge of each device taken from the tube.
A junction transistor is a bi-polar device whose central base region lies between emitter and collector regions and is separated therefrom by PN junctions. Thus in a junction field effect transistor (J-FET), a bar of one type of semiconductor unipolar material has junctions of opposite types diffused on both sides and interconnected to form the transistor gate. Major categories of junction transistors include grown junctions and allow junctions, as well as diffusion and epitaxal types.
Metal oxide semiconductors (MOS) are semiconductor devices in which an insulating layer critical to the operation of the device is an oxide of the substrate material. For a silicon substrate, the insulating layer is silicon dioxide. Thus a metal oxide semiconductor field effect transistor (MOS-FET) has a gate insulating from its semiconductor substrate by a thin layer of silicon dioxide. Metal oxide semiconductor processes include CMOS, DMOS, NMOS, PMOS and VMOS.
Integrated circuits (IC) made up of MOS transistors can have a higher density of equivalent components than bi-polar or junction transistor integrated circuits. The term "semiconductor device" as used herein is intended to encompass discrete semiconductor components as well as integrated circuits formed thereby.
Electrostatic charges pose a grave threat to the life of semiconductor devices of the junction or MOS type, such as discrete FET's, linear operational IC amplifiers with FET inputs, microprocessors, inverters and comparators. It is well known that when a charge of static electricity that has accumulated in a semiconductor device is discharged across the input circuit thereof or through its sensitive gate, or from a terminal pin to ground, this may cause an internal open or short circuit, thereby effectively destroying the device. Because of their high input impedances, the gates of some FET's, especially MOS types, must be treated very carefully, for they can easily be blown by electrostatic discharges or even by slight currents from an ungrounded soldering iron.
Various techniques have heretofore been employed to cope with the problem of electrostatic discharges in semiconductor devices. One approach in widespread use employs grounding stations for personnel handling the devices. At these stations, "Velostat" wrist straps or conductive materials are coupled to the operator and to his work area, use also being made of conductive mats. In this way, the operator, the conductive mat, the work bench and all other surfaces at the station are connected to ground through resistance paths having ohmic values lying in the range of about 100,000 ohms to one megohm. These paths act effectively as short circuits to static electricity.
The grounding station technique for protecting semiconductor devices from destructive discharges has a number of practical drawbacks. Because a human body can develop as much as 50,000 picofarads of capacitance, a highly-charged semiconductive device coming out of a plastic storage tube can often discharge enough energy to an ungrounded or improperly grounded operator to permanently damage the device. Moreover, human operators create electrostatic discharge paths which differ from person to person, depending on sex, hair characteristics, clothing properties and body chemistry. And when an operator works in an environment having a fluctuating humidity, this, too, influences the effectiveness of ground stabilization. Finally, there is the not uncommon situation where the operator neglects to attach himself to "Velostat" wrist straps. All of these factors adversely affect the reliability of this traditional approach to static protection.
Another technique heretofore employed for static protection of semiconductor devices involves shipping the device in "anti-stat" plastic carrier tubes or storage bags designed to eliminate static charges. This approach is based on the action of a Faraday shield serving as a barrier between the device and static electricity present in the atmosphere. The plastic carrier tube prevents electrostatic charges from reaching the leads of the semiconductor devices.
The anti-static storage tube approach suffers from certain practical disadvantages. First, because the devices in the "anti-stat" tubes are generally in loose form, in the course of transit the devices may be shaken up, thereby generating triboelectric charges. Second, since it becomes necessary at some point to remove the devices from their protective tubes and place them into a mechanical autohandler or auto inserter either for testing or automatic insertion in printed circuit boards, electrostatic protection is lost at this crucial point. And when the devices are taken from their shielded tube to be handled by an operator, one again exposes the devices to deleterious charges in an uncontrolled environment.
Autohandler problems deserve special consideration. Typical of such handlers is the "IC Test Handler with Digital IC Tester," marketed by Daymarc Corporation of Waltham, Mass. In such handlers, magazines are plugged into feed mechanisms which are almost invariably made out of metal and therefore are usually at ground potential. In this situation, one has no control over the electrostatic environment presented by the autohandler. If a particular semiconductor device has acquired a substantial electrostatic charge in the course of transit, when it enters the handler it is abruptly discharged thereby, as a consequence of which sensitive gates in the semiconductor device may be blown up.
Still another technique heretofore employed to prevent destructive electrostatic discharges in semiconductor devices makes use of ion generators. These ionize the air in the work area, the ionized air being circulated to render the atmosphere around the semiconductors more conductive. This approach is intended to raise the leakage rate of static charges from the semiconductors to ground, thereby discouraging the build-up of such charges on the package surfaces of the semiconductor devices and inhibiting controlled destructive discharges.
Two practical difficulties are encountered in this ionization technique which ordinarily employs a nuclear source or a corona generator. In the case of a nuclear source, in order to create a localized atmosphere having a concentration of ions at a level sufficient to render the atmosphere conductive, one requires a powerful nuclear flux of beta or alpha particles. But radiation densities at levels sufficient to provide acceptable leakage paths for the electrostatically-charged semiconductors may represent unacceptable and hazardous levels of radiation to operators working in the same environment.
If, on the other hand, one uses a high-voltage corona generator to ionize the atmosphere, the resultant corona discharges may at the same time impart electrostatic charges to the semiconductor devices, particularly when the resistivities of the packages of the devices and the resistance of the air are in proper balance.
The present invention recognizes that to solve the problem of destructive static discharge, one must focus on the transfer region wherein semiconductor devices coming out of thier storage tubes and carrying electrostatic charges then proceed to make contact with personnel at a work station or enter a manual or autohandler where they make contact with rails and terminals. As explained previously, if personnel make contact with the devices, the uncontrolled capacitance of human operators coupled with the effects of electrostatic phenomena associated with clothing fibers and prevailing atmospheric conditions may be conducive to an electrical discharge of sufficient intensity to destroy the semiconductor. This may occur even when the discharge is not actually felt by the person handling the device.
If, on the other hand, the device from its storage tube is transferred to the rail or track of a typical autohandler connected to a test computer, then a different situation exists. Let us assume, for example, that the semiconductor device is an integrated circuit in a standard dual-in-line (DIP) package whose leads are engaged by Kelvin contacts mounted on the track receiving the device.
These Kelvin contacts on the track are connected to the pin electronics of a computer-controlled IC test system to determine the condition of the integrated circuit before it exits from the autohandler and is put to use. Since the pin electronics are generally fairly low-impedance bipolar transistors, the moment the Kelvin contacts are clamped onto the leads of the IC device, the device sees a very low impedance path to ground. This acts to abruptly discharge whatever static charge is carried by the device, with possibly damaging effects thereon. In some cases, this discharge may also destroy the associated bipolar transistors in the test computer.
The need exists to provide means preventing destructive electrostatic discharges from taking place in the course of the transfer of a semiconductor device from its storage tube to a processing station, whether this station takes the form of a mechanical handler or inserter, or personnel who physically handle the device.